I have children. I would say it's not a legitimate gripe of the alternative marriage crowd, and not because I benefit... (Also I've never heard it associated with the alternative marriage crowd, just a small group of people who don't want children, whether they're married or not. Usually they're the same group that obsesses over "overpopulation")
The purpose of progressivity in taxes is to ensure that taxes aren't so high as to make it difficult to live. On lower incomes, taking 20% of someone's income is essentially cutting into their living expenses. On higher incomes - well, you're cutting into your "Plasma TVs for every house in the room needing replacing because they're a week off" budget. OK, I'm being facetious on the latter, but you know what I mean.
Now, if your income supports multiple people, your cost of living will be higher. And it gets worse if the people you support means that some people who could potentially be wage earners can't do that because they have to be around supporting the others - or else you'd need to spend money on supporting the others while person who could be a wage earner or a caregiver takes on the former role.
And that describes having children exactly. You either have two working parents with high expenditures on childcare, or one working parent and no income from the caregiver parent.
Any reasonable society would recognize that the cost of living for the 2+1 group is higher than the 2 group, and set progressivity in the tax rate to reflect that.
Now, I know some people see it as a subsidy because they don't want children and don't see why other people should get a subsidy (etc), but it isn't one. Trying to get as much money out of parents as a non-parental couple is ultimately (1) a getting-blood-out-of-a-stone situation and (2) going to result in malnourished, poorly educated, badly brought up kids. Moreover, kids don't stay kids. Eventually they grow up. And they'll pay taxes.
It shouldn't bother anyone who's supportive of gay marriage. Taxes should be practical, and in any case, gays and lesbians who adopt (or use IVF and a donor, etc) will have exactly the same "benefits" (relief from taxes isn't really that, given it comes at the cost of the aforementioned higher cost of living, the real benefit is hearing your kid laugh, and being able to watch and guide them as they grow up, but I shouldn't have to say that...)